In the past, there are many proposals for improving the corrosion resistance of steel materials by forming metal coating film layers and resin coating film layers on the surfaces of steel materials.
For instance, chromate layers formed by means of a chromate solution containing hexavalent chromium ions show superior anti-rust properties and adhesion (primary adhesion and secondary adhesion) to resin layers; and thus, the corrosion resistance of steel materials that have zinc type coating films has been improved by forming a composite coating film structure that includes a resin coating layer and a chromate layer containing hexavalent chromium on the surfaces of such steel materials that have zinc type coating films.
In the industry, as described above, desired metal layers, e.g., Zn plating layers (zinc type coating films) are formed between the surfaces of steel materials and the chromate layers from the standpoint of improving the corrosion resistance, etc.
However, hexavalent chromium type chromate layers contain a hexavalent chromium ion that is a substance which places a burden on the environment. Accordingly, the effect exerted on the environment when such steel materials are discarded is a social problem.
In European legal regulations (ELV order: End of Life Vehicle), a complete ban on the use of automobile parts that contain hexavalent chromium is planned to take effect in the year 2005.
As a response measure, the formation of chromate layers (hereafter referred to a trivalent chromate layers) that use a chromate solution consisting of trivalent chromium ions instead of hexavalent chromium has been practiced in order to suppress the burden on the environment.
For example, in corrosion-resistant pipe materials that have been proposed, a Zn plating layer is formed on at least the outer circumferential surface of the pipe material, a trivalent chromate layer is formed on top of this plating layer, and a resin covering layer consisting of a polyvinyl fluoride resin, etc., is formed on top of this chromate layer with a primer layer consisting of an epoxy resin, etc. being interposed.
However, in corrosion-resistant pipe materials in which a resin-covering layer is combined (formed as a composite) with a trivalent chromate layer instead of the hexavalent chromium layer, the adhesive strength between the trivalent chromate layer and resin layer is not very strong. Consequently:                Both interfaces tend to peel as a result of end molding or bending of the pipe material (this characteristic will be described below as a characteristic that is associated with primary adhesion); and        In automotive piping, in cases where damage is generated in the resin layer as a result of flying rocks or mud during operation of the vehicle, corrosive elements such as moisture or salts, etc., easily invade the interface between the resin layer and metal layer so that the corrosion rate is accelerated (this characteristic will be described below as a characteristic that is associated with secondary adhesion).        